1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a belt member, a transfer unit including the belt member, an image forming apparatus including the belt member, and a method of evaluating the belt member.
2. Discussion of the Related Art
Full-color image forming apparatuses for electrophotographic printing generally perform either a direct transfer operation or an indirect transfer operation. In the indirect transfer operation, a toner image formed on an image carrier that contacts a belt is transferred onto an outer circumferential surface of the belt in an electric field supplied by a transfer bias unit, in an operation that is referred to as primary transfer. Then, the toner image retained by the belt is transferred onto a transfer member or a recording medium conveyed along the outer circumferential surface of the belt, in an operation referred to as secondary transfer. Through the primary and secondary transfers, ultimately a full-color toner image is formed on a recording medium.
For an image forming apparatus to perform the above-described primary and secondary transfers, a belt or an intermediate transfer belt having medium-resistance in volume resistivity is commonly employed. Such a medium-resistance intermediate transfer belt is cost-effective since it is generally used without a belt charge eliminator for eliminating residual charge remaining on the intermediate transfer belt.
When such an intermediate transfer belt having medium resistance is used for image forming, the transfer bias applied for primary transfer charges an outer circumferential surface of the intermediate transfer belt to a given electric potential. Soon, the charge forming the electric potential is gradually bled from the intermediate transfer belt via supporting rollers that serve as supporting means contacting an inner circumferential surface of the intermediate transfer belt, and thus the electric potential of the intermediate transfer belt decreases to close to 0V. Thus, the intermediate transfer belt having medium resistance does not hold residual charge, and therefore irregularities such as afterimage caused by residual charge can be prevented.
By contrast, an intermediate transfer belt having high resistance can hold an electric potential that is created during one primary transfer until a subsequent primary transfer. In this case, it is difficult to form a desired electric field in the subsequent primary transfer due to the presence of residual charge on the intermediate transfer belt. Accordingly, the electric field in the subsequent primary transfer may be different from that in the previous primary transfer, and therefore an intermediate transfer member having high resistance may need to be used with a belt charge eliminator, addition of which can cause an increase in the cost of an image forming apparatus.
As the electric potential on the medium resistance intermediate transfer belt decreases close to 0V, an electric potential at a portion of the intermediate transfer belt that corresponds to a background portion of an image, where no toner image is formed, and an electric potential at a portion of the intermediate transfer belt where a toner image is formed may differ significantly. This is because, when a color toner image is formed by sequentially superimposing single-color toner images on top of each other, the electric potential of toner on a surface of the color toner image may be high, and as a result the charged toner image (more precisely, the charged toner particles) may be attracted to the outer circumference of the intermediate transfer belt due to localized differences in potential. As a result, some toner particles on the color toner image may scatter to the outer circumference of the intermediate transfer belt, which can adversely affect image quality.
The above-described toner scattering is particularly noticeable in full-color image forming and is regarded as one of the causes of image deterioration or irregularity, such as image background contamination and ink bleed on text.
Further, the medium resistance intermediate transfer belt has low electrical withstand voltage. Therefore, when a toner image is transferred at a secondary transfer nip from the intermediate transfer belt to a recording medium, spot-like discharges may occur in the secondary transfer nip that generate hollow defects or white spots on the toner image transferred onto the recording medium. Particularly with low ambient humidity, high resistance for duplex copy, and high voltage for the secondary transfer, hollow defects or white spots can appear on the toner image.
For example, one conventional image forming apparatus includes a multi-layer intermediate transfer belt composed of a high-resistance surface layer that forms an outer circumferential surface for carrying a toner image thereon and a medium-resistance base layer that forms an inner circumferential surface of the multi-layer intermediate transfer belt to which a transfer bias is applied. Such a high-resistance surface layer can provide high charge retention, which can reduce a potential difference between a surface potential of the multi-layer intermediate transfer belt and a charged potential of toner attracted to the outer circumference of the intermediate transfer belt. Thus, the above-described toner scattering can be reduced, thereby preventing a decrease in image quality in development.
Further, the high-resistance surface layer can increase the electrical withstand voltage of the multi-layer intermediate transfer belt, and as a result, the spot-like discharge in the secondary transfer nip can be prevented to avoid white defects in images.
However, a drawback of the conventional composite belt having a high-resistance surface layer and a medium-resistance base layer is that, while good charge retention to prevent toner scattering can be obtained, the electrical withstand voltage is not sufficient to prevent occurrence of the spot-like discharge to produce an image with white spots.
Generally, a composite belt having a high-resistance surface layer on top of a medium-resistance base layer is manufactured to have a given tolerance. The upper and lower limits of the tolerance are determined not only by the belt's quality but also by its manufacturability, such as mass productivity. Therefore, even within the tolerance, the composite belt may have deviations in quality. Therefore, a final determination of such a composite belt is generally made according to a test of characteristics of the composite belt.
However, when testing to evaluate the characteristics of the surface layer of the composite belt, the evaluation is generally made on the basis of the combined characteristics of the base layer and the surface layer. Therefore, it is probable that the evaluation cannot accurately evaluate the characteristics of only the surface layer of the composite belt.
For example, FIG. 1 shows changes in surface resistivity of two test composite belts. A surface resistance of a belt is generally determined according to a resistance measured over an arbitrary period of time, e.g., 10 seconds. However, as shown in FIG. 1, while the surface resistivities of Belt A and Belt B are substantially equal to each other at the end of such measurement time of 10 seconds, thereafter Belt A maintains a substantially constant surface resistivity whereas the surface resistivity of Belt B increases with time. Consequently, the measurements obtained at the 10-second point do not provide an accurate evaluation of the characteristics of the surface layer of the conventional composite belt.
Such inaccurate evaluation of the characteristics of the surface layer of a conventional composite belt produces variations in the quality of the composite belt. Therefore, even though good charge retention of the composite belt having a high-resistance surface layer mounted on a medium-resistance base layer is obtained to prevent an occurrence of toner scattering in transfer, the electrical withstand voltage remains insufficient to prevent the occurrence of the spot-like discharges, and therefore images with white spots are generated.